The present invention relates to a voltage generator in a flash memory device, and more particularly to a generator for supplying a desired voltage irrespective of temperature of the flash memory device, and for supplying a supply voltage when reading data and verifying program operation.
Generally, the performance of program operations, erasing operations, reading operations, verifying operations, in a flash memory device depend on threshold voltage distribution. The threshold voltage distribution changes in accordance with a voltage when programming or erasing a cell in the flash memory device. Accordingly, when a reading operation or a verifying operation is performed by applying a constant voltage to a gate of a selected cell, error data may be read or verified. This is because the margin between a threshold voltage of a memory cell and the voltage applied to the gate of the memory cell is reduced.
FIG. 1 is a graph illustrating the common threshold voltage distribution of cells. The x-axis and y-axis of the graph indicate the threshold voltage and the number of cells, respectively.
A reference voltage may be fixed when programming a cell. Thus, increasing the operating temperature of the flash memory device also increases the threshold voltage of a programmed cell, as shown by A of FIG. 1.
When reading data in the cell after the cell is programmed, an increased temperature of the flash memory device reduces the threshold voltage of the cell, as shown by A′ of FIG. 1. Accordingly, error data may be read due to a difference between a read voltage and the voltage of the programmed cell when reading data in the cell. In other words, when a cell is programmed at a low flash memory device temperature and when the cell is read at a high flash memory device temperature, the programmed cell may be misread as an erased cell. Likewise, the lower the temperature of the flash memory device, the lower the threshold voltage when programming the cell, as shown by B of FIG. 1.
When reading data in the cell after the cell is programmed, a low flash memory temperature increases the threshold voltage, as shown by B′ of FIG. 1. Accordingly, when a cell is programmed at a high flash memory device temperature and when the cell is read at a low flash memory device temperature, an erased cell may be misread as a programmed cell.